If there is a secret to the art of the really great conductor, then Mariss Jansons thinks it is preparation. "I prepare," he says, "for a very long time. It is my way." In person, and even on the platform, the Latvian-born conductor appears direct and spontaneous. Few maestros can be as relaxed, as sociable and as lacking in airs. But the apparent ease on the podium and at the lunch table ("I like to talk and I like to eat; let's do the two together") is built on a rock of private, intensive study.
Jansons, 61, is in the early part of a year in which an already distinguished career is taking another immense step. Until now, he has specialised in the precious art of building a relatively modest orchestra into one of which the world is forced to take notice. For 23 years, he conducted the Oslo Philharmonic, doing for that orchestra what Simon Rattle did for the City of Birmingham Symphony. More recently, Jansons has been in charge of the Pittsburgh Symphony, always the bridesmaid among the lustrous American bands but now, as Proms listeners found out last August, in two memorable Jansons concerts, one to rank with the very best.
But those phases are over. Now Jansons is taking his baton to orchestras that have nothing to prove. Since the autumn, he has been in charge of the Munich-based Bavarian Radio Symphony Orchestra, one of the greatest of all German orchestras, which he brings to London for the first time on Monday. Then, in September, Jansons takes over from Riccardo Chailly at the helm of the Concertgebouw Orchestra in Amsterdam. Jansons has finally arrived at the summit of his mysterious profession.
He is clearly revelling in the experience. "This Bavarian orchestra is unbelievably good. You know, you can say that such and such an orchestra has technical perfection, or that another has supreme musicality, while another has a uniquely interesting sound. Here everything is on the highest level. The sound is wonderful. Technique is great. Ensemble, musicality, involvement, style, feeling - all the same. It is such a pleasure to have such an instrument. It is like being presented with a Rolls-Royce to drive."
Jansons says he knew straight off that Munich was the place for him. "I came as a guest conductor. And after the first hour of rehearsal, in the interval, I said to my wife that this is an orchestra I want. Like me, they want to do music with passion, with involvement, with an enormous feeling of atmosphere, of imagination. They are real musicians."
One of Jansons' most striking skills is his ability to take a well-trodden piece and serve it up fresh, new and thrilling. He did that with Dvorak's Ninth with the London Symphony Orchestra at the 2002 Proms, and with Tchaikovsky's Fourth with his Pittsburgh orchestra last summer. Expect similar revelations when he brings the Bavarians to London on Monday with two of the most battle-hardened warhorses in the repertoire - Beethoven's Fifth and Berlioz's Symphonie Fantastique - and when they return for two Proms later in the year.
Can he explain the secret formula that enables a select group of men and women to turn a good performance into something memorable and liberating?
"I don't know if I succeed always, but I can only tell you what I try to do," he says. "I was always a perfectionist. My teachers were perfectionists - my father (the conductor Arvid Jansons), Mravinsky, Swarowsky and Karajan. But with the years I also began to ask myself: what makes a great performance?
"Well, you can do everything fine. You can prepare right. You can get the texture as it is written in the score. Everything is on time. Ensemble is OK. Even the sound. But there is something else that is on a higher level. I call it the cosmic level. For me, notes are signs, like words, that convey associations. So the question is, what is behind the notes? What is the meaning and the atmosphere of the notes? This is the inner world of music, and if you can enter this world this takes the performance on to a much higher level.
"A conductor can achieve this through inner energy, or with the hands, or sometimes with words. It is easy to tell an orchestra to play a crescendo or an accent, but if you can create the right image, content and atmosphere, then they will play in a completely different and more meaningful way."
That Jansons is conducting at all is something of a miracle. In 1997, while conducting a performance of La Bohème in Oslo, he had a major heart attack and almost died. He was fitted with a defibrillator, took several months out to recuperate and then resumed his schedule. "I don't drink, I don't smoke, but I do like desserts," he says, tucking in to a Bavarian berry and cream pudding.
Jansons is not just changing jobs. He is also changing repertoire. His overall aim with any orchestra, he explains, is to broaden the repertoire. In his own case, though, there is a shift in priorities that seems to reflect his own life journey.
"When I worked in Leningrad, I did so much Russian music, especially on tours. Then I decided to stop doing Russian music almost at all - except Shostakovich. Now I don't do Tchaikovsky much, nor Prokofiev, nor Rachmaninov. I stopped for two reasons. First, because my name was becoming identified simply with Russian music. And second, because artistically I thought, 'Enough, I should go more in another direction.' But Shostakovich I will always conduct. I feel so close to him."
Now, as he begins to bestride the central European music world, Jansons is increasingly drawn back to what used to be the core repertoire of the modern symphony orchestra. "I am now concentrating on classical music," he explains. "Composers such as Haydn, Mozart, Beethoven, Schubert and Mendelssohn. And the time has come for Bruckner. I used to regard him as a very great specialism, almost untouchable. Now, I feel much closer to him.
"The one thing I really want to do is to play more Haydn symphonies. I think it is very necessary for orchestras to play Haydn, especially as they have had less opportunity in recent years to play classical period music."
But how can this be done after a quarter of a century in which performance practice in Haydn and Mozart has been completely transformed? Is Jansons implying that he will try to roll back the clock and perform classical symphonies the way that Toscanini and Furtwängler might have done?
"No. I think the pendulum is beginning to swing back, but to a new place. For some years, symphony orchestras have not been confident to play this music. Their approach has been criticised as too heavy and it has become a widespread view that Haydn and Mozart can only be played by chamber orchestras and on original instruments.
"But we are moving towards a synthesis of this and the more romantic approach of the past. I don't deny the impact of the modern approach. It was enormous. It helped a lot. But it is essential for the modern symphony orchestra to reclaim that music."
Could his wish to reclaim go back to Bach, whom an earlier generation of conductors did not hesitate to programme (albeit in arrangements which would appal modern scholars)? "I think orchestras should play Bach, yes," he says. "But Bach is a very special world. I would need to prepare for a very long time. Just to open the score and conduct would be no use. You have to learn more than I know before you enter this world."
When he moves to Amsterdam, Jansons will also fulfil a long-nurtured wish to return to the opera house, in which the Concertgebouw also plays. "I practically grew up in the opera house," he says, "and it is my passion." He is cagey about his repertoire wish-list - "Verdi, Puccini and others" is all he will say. Wagner, however, is not a priority. "I would answer the same as I did about Bach. I would need time to study and read. I must always be completely inside a piece, and completely prepared."
There it is again, that iron self-discipline. As his career turns this significant corner, it remains the interior key to the flamboyant outer image that Jansons projects on the podium. His American years are now behind him; he dislikes the travel. So, too, the years on the periphery of European music. From now on, he is at the centre of things.
"I think perhaps this is the way it happens," Jansons reflects. "For conductors, it gets better as you get older. Conductors reach their peak in their 60s and 70s, I think."
· Mariss Jansons conducts the Bavarian Radio Symphony Orchestra on Monday at the Royal Festival Hall, London SE1. Box office: 020-7960 4203.
